Sawtooth Activity Research Articles

Behaviors of Electron Heat Transportation in HT-7 Sawtoothing Plasma

It is found that in HT-7 ohmic plasma, main energy loss comes from electron heat conduction, hence quantitative data of electron heat diffusivity is a very important issue for investigation of electron heat transportation behavior in different target plasmas so as to get high performance plasma. A time-to-peak method of the heat pulse propagation originating from the sawtooth activity on the soft x-ray intensity signal has been adopted to experimentally determine electron heat diffusivity χeHP on the HT-7 tokamak. Aiming to improve the signal-to-noise (S/N) ratio of the original signal to get a stable and reasonable electron heat diffusivity χeHD value, some data processing methods, including average of tens of sawteeth, is discussed. The electron heat diffusivity χeHP is larger than χePB which is determined from the balance of background plasma power. Based on variation of the measured electron heat diffusivity χeHP, performances of different high confinement plasmas are analyzed.

Observation of Magnetohydrodynamics Instabilities in IonBerstein Wave and Lower-Hybrid-Current Driving Synergetic Discharges on HT-7 Tokamak

The normalized performance indicated by the product of βNH89>2 was achieved by a combination of the lower hybridcurrent driving (LHCD) and the ion Berstein wave (IBW) heating in theHT-7 tokamak. More than 80% of the plasma current was sustained by theLHCD and the bootstrap current. Large edge pressure gradients wereobserved. The magnetohydrodynamic (MHD) instabilities were often drivento terminate the discharge or reduce the discharge performance, whenthe IBW resonant layer was near the rational surface. The resonantlayer of the safety factor q = 2 is located at 0.6 awith a = 27 cm being the minor radius. The width of magnetic island(the poloidal mode number m = 2) was about 2 cm. The plasmaenergy was reduced quickly by 30% by MHD instabilities. The behaviourof MHD instabilities is reported. A large sawtooth activity (m = 1)was observed before inducing MHD (m = 2).

Chapter 4: High-Density Regimes in the FTU

High-density plasmas (no ~ 8 × 1020 m-3) achieving steady improved core-confinement have been obtained in the Frascati Tokamak Upgrade (FTU) up to the maximum nominal toroidal field (8 T) by deep multiple pellet injection. These plasmas exhibit also high purity, efficient electron-ion coupling, and peaked density profiles sustained for several energy confinement times. Neutron yields in excess of 1 × 1013 n/s are measured, consistent with the reduction of the ion transport to neoclassical levels. Improved performance is associated with sawtooth stabilization that occurs when the pellet penetrates close to the q = 1 surface. In this regime, impurity accumulation can be prevented if a slow sawtooth activity is maintained. Experiments aimed at obtaining radiation-improved modes at high field have also been carried out using neon injection. The observed increase of the average density, with respect to the reference discharge, is significantly larger than the contribution of Ne. The neutron yield increases also by a factor of 3 to 6, and the energy confinement time increases by a factor up to 1.4.

Snake perturbations during pellet injection and LHCD in the HL-1M tokamak

Excitation of snake perturbations has been observed in the core region of pellet-fuelled HL-1M plasmas when the pellets cross the surface with a q value of 1. It is observed that the snake oscillations have an m = 1, n = 1 helicity with quite a long lifetime. A detailed comparison has been made between the locations of the q = 1 surface and the snake oscillation. Through measurements of the plasma q-profile by means of multi-exposures with a CCD camera during pellet ablation, and investigation of the pellet ablation process, possible mechanisms for the formation of the snake oscillation are discussed. In addition, a large, long-lived snake-like oscillation is frequently observed in lower-hybrid current driven (LHCD) discharge in which the sawtooth has been stabilized early in the discharge. There is evidence that such a perturbation is due to impurity accumulation during sawtooth-stabilization, and good performance with peaking profiles after LHCD is limited by magnetohydrodynamic instabilities including sawtooth and snake activities in HL-1M plasmas.

Particle transport and density profile analysis of different JET plasmas

Over the last two years, several experiments relevant for the study of particle transport and density profile evolution, have been performed at JET. They can be classified as stationary discharges with and without central particle source due to the beams, quasi-stationary discharges with deuterium gas puffing, deep pellet fuelled discharges and discharges perturbed by cold pulses obtained by shallow pellet injection. All these experimental scenarios have been simulated by means of the JETTO transport code, employing different transport models: purely empirical models and the semi-empirical mixed Bohm/gyro-Bohm transport model, both with the addition of different theory-based expressions for the anomalous particle pinch and the first principle Weiland transport model. The coefficients used to scale the pinch velocity in the purely empirical and in the mixed Bohm/gyro-Bohm model have been varied from shot to shot. In this paper, the results of the simulations are presented. The main conclusions are that, for the cases studied in this paper, the sawtooth activity is the main particle transport mechanism in the plasma centre (r/a ⩽ 0.5). Nevertheless, to reproduce the density profile in the gradient zone (0.5 ⩽ r/a ⩽ 0.9), an anomalous pinch seems to be necessary, at least for L-mode plasmas. This anomalous convective flux is well reproduced by the off-diagonal elements of the transport matrix given by the Weiland model.

Role of sawtooth in avoiding impurity accumulation and maintaining good confinement in JET radiative mantle discharges

Impurity injection in the JET ELMy H-mode regime has produced high-confinement, quasi-steady-state plasmas with densities close to the Greenwald density. However, at large Ar densities, a sudden loss of confinement is observed. A possible correlation between loss of confinement and the observed MHD phenomena, both in the core and in the edge of the plasma, was considered. The degradation in confinement coincided with impurity profile peaking following the disappearance of sawtooth activity. In addition, impurity density profile analysis confirmed that central MHD modes prevented impurity peaking. Experiments were designed to understand the role of sawtooth crashes in re-distributing impurities. Ion-cyclotron radio frequency heating was used to control the central q-profile and maintain sawtooth activity. This resulted in quasi-steady-state, high-performance plasmas with high Ar densities. At and high Ar injection rates, quasi-steady-states, which previously only lasted <1τE, were now maintained for the duration of the heating (Δ t ∼ 9τ E). The increased central heating may have an additional beneficial effect in opposing impurity accumulation by changing the core power balance and modifying the impurity transport as predicted by neo-classical theory.

Signal processing techniques based on singular value decomposition applied to modulated ECH experiments

The analysis and interpretation of perturbative modulated electron cyclotron heating (MECH) discharges for power deposition localization using different diagnostics, such as electron cyclotron emission (ECE) and soft x-ray (SXR) measurements, are presented. In particular, the experimental determination of the MECH power deposition is investigated, which is a fundamental requirement, for example, for heat transport studies. The most important problem is related to the coupling between the MECH and the sawtooth activity of the plasma, which disturbs both ECE and SXR measurements. Several techniques have been adopted to circumvent this difficulty. In particular, the singular value decomposition (SVD) and the generalized singular value decomposition (GSVD) have been tested. However, both methods are incapable of treating the problem correctly, which leads to potential misinterpretation of the results. A new method based on system identification using the SVD (SI-SVD) is developed and applied. This method, within reasonable limits induced by the assumption of linearity, is capable of simultaneously separating the MECH from the sawtooth contributions to both ECE electron temperature measurements and SXR emission measurements. Such a method is in particular applied to a neutral beam injection (NBI) heated ASDEX Upgrade discharge in which MECH is added in order to analyse electron heat transport in a mostly ion-heated plasma. Since the NBI heating is also partly modulated with short pulses which coincide with the sawtooth crashes to improve their stability, both the MECH and the NBI deposition profiles are determined. Moreover, treating the signals with the SI-SVD procedure enables a study of the plasma dynamic response also at higher MECH harmonic numbers.

The Study of Heat Pulse Propagation on HT-7 Tokamak

The propagation of heat pulses originating from sawtooth activity has been studied on the HT-7 tokamak. Two theoretical models are used for determining electron heat diffusivity from the experimental data measured by a diagnostic system of soft x-ray diode-array. The results show that one model called `dipole model' is more suitable for HT-7 tokamak. In order to improve the signal-to-noise (S/N) ratio of the original signals, over a few tens of sawteeth are averaged to generate nice waveforms. The space-time evolution is found to be diffusive in character, which is consistent with the theoretical model. The electron heat diffusivity determined from heat pulse propagation is larger than that determined from background plasma power balance. The variation of χe in different discharge phases has been also discussed.

Density profile control with current ramping in a transport simulation of IGNITOR

Current ramping to achieve reversed shear confinement enhancement and peaked density profiles are important for achieving ignition conditions in the high-field tokamak IGNITOR [Coppi et al., Phys. Scri. 45, 112 (1992)]. Previous transport simulations used either fixed density profiles or obtained flat density profiles leading to a conclusion that a mechanism for peaking the density profile is required for ignition. In this paper an enhanced particle confinement regime that produces ignition before the sawtooth activity begins is explored. It is shown that fast current ramping is a general scheme leading to density profile peaking. In these simulations, peaked density profiles result from the formation of internal transport barrier due to reversed magnetic shear, which is produced by controlled plasma current and volume-averaged density ramping. Such a programmed Ohmic heating scheme is demonstrated to be an effective approach to achieve ignition of a deuterium-tritium plasma.

Sawtooth activity of the ion cloud in an electron-beam ion trap

The dynamics of an ensemble of highly charged Ar and Ba ions in an electron-beam ion trap (EBIT) was studied by recording time-resolved x-ray spectra emitted from trapped ions. Sawtoothlike signatures manifest in the spectra for a variety of EBIT operating conditions indicating a sudden collapse of the ion inventory in the trap. The collapse occurs on a time scale of approximately 100 ms and the evolution of the sawteeth is very sensitive to parameters such as electron-beam current and axial trap depth. Analysis of the measurements is based on a time-dependent calculation of the trapping process showing that sawtooth activity is caused by the feedback between the low-$Z$ argon and high-$Z$ barium ions. This unexpected behavior demonstrates the importance of nonlinear effects in electron-beam traps containing more than a single ion species.

Observations of the snakelike oscillation phenomenon on HT-7 tokamak

Snakelike oscillation phenomenon has been frequently observed on HT-7 superconducting tokamak in various target plasmas. The longest lifetime snake, found in the pellet-fuelled discharge, can survive ten big sawtooth collapses and persist intermittently for a long lifetime of 53.7 ms more than three times the particle confinement time τP. The Ohmic spontaneous snake frequently occurs in the high-density discharge after wall siliconization or wall lithium-containing siliconization when the central line-averaged density is over a threshold value of 3.5×1013 cm−3, appears after the onset of the large sawtooth event and persists intermittently for a comparable long lifetime to that of the pellet-induced snake. In the ion Bernstein wave (IBW) heated plasma, while the antenna frequency ω is set at 30 MHz and the second harmonic deuterium cyclotron resonance layer is close to the sawtooth inversion radius, spontaneous snake oscillation easily occurs in the later phase of the IBW heated plasma or after switching off IBW power. In the low hybrid current drive plasma, negative snake appears and terminates silently. Suppression of the sawtooth activity and the impurity accumulation in the central region of the plasma are found to play a significant role in the occurrence of the spontaneous snake oscillation. In this paper, behaviours of the pellet-induced snakes have been summarized. Characteristics of various spontaneous snakes have also been presented and discussed.

Sawtooth characteristics in ohmically heated HT-7 plasma

The sawtooth activity in an ohmically heated plasma on the HT-7 tokamak has been investigated experimentally using soft x-ray (SXR) diode arrays, magnetic probes and other diagnostics. Its behaviour and occurrence are correlated closely to the discharge conditions such as the electron density and wall condition have been summarized. Experimental properties of two types of double sawtooth and monster sawtooth activities are also presented and discussed. The set up of the HT-7 tokamak and the main diagnostics available are also briefly described.

Sawtooth-like X-ray emission observed in EBIT

The evolution of a mixture of highly charged Ar and Ba ions was measured in an electron beam ion trap (EBIT) by recording the characteristic X-ray emission from trapped ions. A special feature in the spectra are sawtooth-like intensity variations caused by a periodic collapse of the ion inventory in the trap. The effect requires favorable conditions to become present and is very sensitive to the trapping conditions. Analysis of the measurements is based on a time-dependent calculation of the trapping process. Simulations show that sawtooth activity results from the feedback between the low-Z Ar and high-Z Ba ions (Hopf bifurcation). Sawtooth spectra open up a spectroscopic method to test theoretical EBIT models and probe the dynamics in ion traps and sources.

Magnetohydrodynamic Stability of Improved Confinement Plasmas in JT-60U

Progress in the understanding of magnetohydrodynamic (MHD) stability is summarized on JT-60U tokamak discharges with improved confinement such as the (hot-ion) H-mode, high-βp mode, high-βp H-mode, and reversed shear discharges. Transport barriers, which are essential for the improved confinement, play key roles in the local and global MHD stability owing to the local large pressure gradient and the related bootstrap current. Disruptive β limits of these discharges are consistent with theoretical ideal kink-ballooning stability limits with low toroidal mode numbers n. Achievable β limit is improved by broadening of the pressure profile with high plasma internal inductance, plasma shaping, and wall stabilization. Edge localized modes (ELMs) and barrier localized modes (BLMs), which are associated with edge and internal transport barriers, respectively, are analyzed carefully. Resistive interchange modes with n ≤ 3 are excited in the negative shear region in reversed shear discharges with the internal transport barrier and lead to major collapse occasionally through nonlinear coupling with a tearing mode in the positive shear region. MHD characteristics of low m/n (m: poloidal mode number) tearing modes, which are attributed to the neoclassical tearing mode, are investigated. Stabilization of tearing modes and control of sawtooth activity are demonstrated using the fundamental O-mode electron cyclotron wave injection. Resistive wall modes associated with current-driven and pressure-driven low n external kink modes are identified.

Sawtooth wave density analysis during REM sleep in normal volunteers

Objective: Sawtooth waves (STW) are a characteristic EEG feature of REM sleep but their source and function are unknown. We previously reported stereotypical properties of STW at stage REM onset, and alterations in bulbar postpolio syndrome. This study analyzes STW features throughout REM, in order to test the hypothesis that sawtooth wave activity may be predictable and have a consistent relationship across REM periods. Methods: Twenty polysomnographic recordings were scored for occurrence, duration, and frequency of STWs. STW density was calculated based on the number of bursts/min REM and duration of STW activity/min REM. The density measurements were statistically analyzed to assess for differences across REM periods. Results: STW density mean was 0.97 bursts/min REM (95% CI [0.85, 1.09]); 6.85 s/min REM (95% CI [5.95, 7.76]). STW frequency range was 1.5–5 Hz, mean 2.5 Hz. STWs occurred in bursts with a mean duration of 7 s (range 2–26 s). There was a lower density of bursts of STW activity per minute in the first REM period compared to the second, third, and fifth cycles. Conclusions: This study reports STW density characteristics throughout REM sleep in normal subjects. Our density measurements suggest a difference in STW activity between the first REM period and later periods. Analysis of STW and related phenomena may increase the understanding of REM sleep mechanisms and may be useful to evaluate brainstem function during normal and pathological sleep.

Effect of suprathermal electrons on central plasma relaxation oscillations during localized electron cyclotron heating on the HL-1M Tokamak

In this paper we discuss the typical behavior of HL-1M tokamak plasmas during ECRH. A large variety ofsawtooth types have been observed at different ECH power deposition locations. The observed central MHD activities detected by the soft x-ray diode array include saturated sawtooth, partially saturated sawtooth, double sawtooth, and strong m=1 bursts superimposed on the ramp phase of sawtooth. Complete suppression of sawtooth is achieved during ECRH, when the heating power is applied on the high-field side of low-density plasmas, and exceeds a threshold value of power. The m=1 bursts riding on the ramp phase of sawtooth can only be excited when the ECRH location is near the q=1 surface at the high field side. The conditions under which the various relaxation activities are produced, or suppressed, are described. Experimental results imply that the energetic electrons, generated during ECRH, are responsible for the modification or stabilization or excitation of the instability. Near the q=1 surface, while the passing electrons play a roleto reduce the shear and tend to stabilize the sawtooth activity, the barely-trapped electrons play a role to enhance or drive aninternal kink instability.

Influence of Superathermal Electrons on Central Plasma Relaxation Oscillations During Localized Electron Cyclotron Heating on the HL-1M Tokamak

During initial studies of ECRH in the HL-1M tokamak, non-standard central MHD activities, such as saturated sawtooth, partially saturated sawtooth, double sawtooth, and the strong m = 1 bursts have been observed while changing the heating location, the ECRH power, the plasma density. Complete suppression of sawtooth is achieved for the duration of the ECRH, when the heating power is applied on the high-field side of low-density plasma, and exceeds a threshold value of power. The m = 1 bursts riding on the ramp phase of sawtooth can only be excited when the ECRH location is near the q = 1 surface on the high field side. The conditions under which the various relaxation activities are produced or suppressed are described. Experimental results imply that the energetic electrons generated during ECRH are responsible for the modification/or stabilization/or excitation of the instability. Near the q = 1 surface, the passing electrons play the role of reducing the shear and tending to stabilize the sawtooth activity, while the barely-trapped electrons play the role of enhancing or driving an internal kink instability.

Dynamics of Plasma during the Formation of a Weak Negative Central Magnetic Shear Configuration Using Counter Neutral Beam Injection in the JFT-2M Tokamak.

In the JFT-2M tokamak, discharges exhibiting good performance have been produced by counterNBI after boronization. An improved core confinement mode under an H-mode edge condition is sustained for -5~~E With H89p - 1.5 and pN - 2.0 at a line-averaged electron density of around 70%-80% of the Greenwald density limit. It is found that the q-profile changes from a monotonic one having qo ~ 1 to a zero or weak negative central magnetic shear configuration having I < qmin ~ qo < 3 during improved core confinement mode using counter-NBI under an H-mode edge condition. The central electron temperature very clearly increases when the sawtooth activity disappears, while the previous result without boronization exhibited a significant degradation of the energy confinement due to the accumulation of impurities. The mechanism governing the improved core confinement mode including the density peaking and/or ITB formation due to negative shear is presently unclear.

Monster Sawtooth Activity in Ohmically Heated HT-7 Plasma

Experimental properties of monster sawtooth activities in ohmic HT-7 plasma are presented in this paper. The monster sawtooth activities belong to global fluctuations and are characterized with a series of large core collapses on SXR intensity traces with a long period, a large amplitude fluctuation and a large inversion radius. However, they emerge without apparent deterioration of plasma confinement and without major plasma disruption. During the events, long partial sawtooth collapses and abundant MHD phenomena are also observed.

Steady-state fully noninductive operation with electron cyclotron current drive and current profile control in the tokamak à configuration variable (TCV)

Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak à configuration variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi-stationary core confinement regime, with little or no sawtooth activity, has been obtained by a combination of off-axis ECH and on-axis CNTR–ECCD.